The present invention relates to a variable wavelength light source capable of changing wavelength of light by using a multiple ring resonator. More specifically, the present invention relates to enabling the variable wavelength light source to obtain output light of a more stable wavelength.
Due to the rapid growth in the Internet, particularly due to increases in use of the large-capacitance contents such as moving pictures and audios, network traffics are drastically increased these days. Wavelength Division Multiplexing (WDM) optical communication can transfer light of a plurality of wavelengths with a single optical fiber, so that it is suited for large-capacitance transmission that is required in the recent networks. Recently, there has been an increasing demand for Dense Wavelength Division Multiplexing (DWDM) communication which can transfer light of still larger number of different wavelengths.
With a WDM communication system, light sources corresponding to each wavelength are required. In accordance with increased capacitance of the networks, still larger numbers of wavelengths are required. Thus, it has become impossible to deal with such state with widely-used fixed wavelength semiconductor lasers. Therefore, a variable wavelength laser light capable of outputting a plurality of wavelengths by a single light source has been expected as a key device of the next-generation optical communication.
As the light sources for widely-used WDM communication, distributed feed-back (DFB) lasers in which diffraction grating is formed along the entire active layer as well as distributed bragg reflector (DBR) lasers in which an active region and a distributed reflector region are formed within a same element have been used frequently. However, the variable wavelength range of the DFB laser is within 3 nm and the variable wavelength range of the DBR is within 10 nm, which are insufficient to be actually applied to a WDM device.
A Sampled-Grating-DBR laser that is obtained by modifying the DBR laser and by disposing a structure in which the diffraction grating cycle is changed periodically can perform variable wavelength operations of the order of several tens of nm by using Vernier effect. However, with the DBR laser in which the active region and the distributed reflector region are formed within a same element, the element size is increased essentially. Thus, it is necessary to go through complicated manufacture steps. Further, long-term electric current impregnation causes defects in the distribute reflector region, so that the proportion of the refractive index fluctuation for the impregnated electric current changes greatly. Therefore, it is extremely difficult to secure the long-term reliability. As described, many variable wavelength semiconductor lasers have been proposed so far. However, those lasers still have many issues to overcome, and it is difficult to put them into practical use.
FIG. 18 is a conceptual chart showing the structure of a variable wavelength laser light source 300 which utilizes the ring resonator depicted in Patent Document 1. With the variable wavelength laser shown in FIG. 18, light oscillated by an SOA (semiconductor optical amplifier) 321 is inputted to ring resonators 301-302, which is reflected by a loop mirror 322 at the terminal end to return to the SOA 321, and is outputted.
At that time, heaters 311-312 attached to the ring resonators 301-202 are turned on to change the temperature of ring waveguides so as to change the effective refractive index. Thereby, the output light can be tuned to a desired wavelength. Thus, long-term property change is insignificant compared to the control method which directly impregnates the electric current. Further, through the use of the ring resonators with slightly different resonance circumference set in a plurality of stages, the resonance peak wavelength of 1 nm interval in a single resonance resonator comes to meet only with one wavelength at a wide interval of several tens of nm due to the Vernier effect.
Therefore, an extremely fine single mode oscillation can be implemented with this wavelength, and the oscillation wavelength can be selected by controlling the heater power of each ring. The variable wavelength laser light using such PLC (Planer Lightwave Circuit) element and SOA is superior in terms of the characteristics and productivity compared to other variable wavelength lasers, and future developments are expected.
In addition, as a patent document related to the variable wavelength light source, Patent Document 2 discloses a technique which attenuates monitor light to an appropriate intensity in an optical waveguide just before a monitoring light-receiving element.    Patent Document 1: Japanese Unexamined Patent Publication 2006-245346    Patent Document 2: Japanese Unexamined Patent Publication 2003-233047
However, the variable wavelength laser formed with a plurality of stages of ring resonators shown in FIG. 18 first determines the ring having the resonance circumference corresponding to ITU (International Telecommunication Union) to be the basis for deciding the oscillation wavelength, and the resonance peak wavelength of the other ring resonator whose resonance circumference length is slightly different is tuned to be the same as the resonance peak wavelength of the reference ring by adjusting the heater power. If the resonance peaks of each ring cannot be adjusted accurately, divergence from the reference ring wavelength and mode jumping (wavelength jumping) are generated.
An object of the present invention is to provide a variable wavelength light source, an optical module, and a manufacturing method of the variable wavelength light source, with which a stable output light wavelength can be acquired without having wavelength jumping and from which unintentional high output power light is not emitted.